The present invention relates to a method and apparatus for testing stretch film.
During the past two decades, considerable developments have been made in the field of wrapping a load with a stretched web of film to package the load. Most noteably, the film web dispenser used in stretch wrapping operations has developed to the extent that it contains a series of rollers which defines a path through which the film web passes so that it can be prestretched prior to being dispensed on the load. Such stretch wrapping apparatus is disclosed, by way of example, in U.S. Pat. Nos. 4,302,920 and 4,418,510 to Lancaster et al., and assigned to Lantech, Inc. These patents are incorporated herein by reference.
With the advent of prestretch, there has been significant confusion of film users and buyers with film manufacturers specifications and sales claims for their films. The use of traditional laboratory film test information has added to the confusion since the properties reported are not suitable for comparing film qualities to determine the best value for shipping unit loads.
Most marketing energy has been concentrated on "stretch-ability," that is, how far the film will stretch on a particular load with a particular type of stretching or prestretching device. Stretchability largely determines the yield of a particular film and thus the cost to wrap a given load with a given number of unit wrap revolutions. Stretchability is measurable on a load by printing "X's" at known intervals, such as every ten inches, on the film supply roll and then measuring the interval after prestretch and wrapping to calculate the actual increase in the film length. Stretchability tests have been done to stretch film and measure percent stretch at film rupture.
The second and often overlooked film characteristic is "relative restretch force." The most important function of stretch film is to hold together as a unit a group of products such as cartons, bags, or bottles. The ability of a film wrap to hold the unit together is largely a function of the unitizing or banding force available to maintain the individual subunit components of the unit together during shipment or material handling.
When an individual subunit shifts during transit of the load and moves away from the unit, the film is restretched so that the film near the shifting subunit is stretched further than it was stretched during the original wrapping process. It is desirable that the force exerted on the load by the film during restretch is as high as possible because a higher force would stop the momentum of subunit at the earliest possible time and produce the best results in terms of holding the package as a unit. A film exhibiting twice as much restretch force as another film may well require only half as many revolutions of film to be applied for equivalent shipment security. Such a film would reduce the cost for unitizing a load substantially.
The force exerted by the film on a shifting subunit of a load is called restretch force. It is further defined as the force required to substantially extend film which had previously been stretched around a load. The force required for initial restretching is substantially dependent upon original wrapping force. As the film is restretched further, the force required for restretching becomes closely proportional to ultimate film strength.
Initial restretch force testing was done by instrumenting a simulated load with an extendable corner. After wrapping, the corner was extended outward and the force required was measured with an electronic load cell. Mobil Chemical demonstrated a variation of this concept at the 1988 PMMI trade show using an extendable pad mounted in the side of simulated unit load. Relative restretch force also has been crudely measured on a load by inserting a rod or pad through a horizontal slit and then pulling a predetermined distance with an electronic or spring scale.
The third important stretch film characteristic is "moldability." This is the ability of the film to conform over irregular objects. Moldability is a critical characteristic when wrapping irregular shaped units such as "order picking" loads of many different sized and shaped products. It also may be critical when the load does not fit the pallet and the film will be required to extend around the sharp corners of pallet deck boards and stringers. This characteristic has been measured and compared by successively wrapping films on a simulated load with several levels of protrusions to determine the failure mode. It also is relatively determinable by repetitively wrapping a certain load with several films and observing failures. Film moldability is dependent upon the balance of several traditionally measured film characteristics including elastic modules, puncture and tear properties.
The first two characteristics, stretchability and restretch force, in combination with film cost are the most important elements in determining relative unitizing cost when comparing two or more stretch film candidates. The third characteristic "moldability" is added to the first two when the unit to be wrapped has a very irregular shape or when it does not fit the pallet.
The need for cumbersome laboratory type equipment such as instron equipment, instrumented loads, or crude field observations to determine these film characteristics has made it very difficult for film users to make an intelligent selection of a film that will insure an optimum film cost per load effectively shipped and handled to its ultimate destination.
Accordingly, it is an object of the present invention to provide a portable test apparatus easily transported and demonstrated on a desk top and a process for using the test apparatus to compare the most important unitizing characteristics of several stretch films to determine the one most likely to unitize a particular product securely through shipment and material handling at the lowest possible film cost.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or maybe learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.